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Laney Graduate School

Emory College

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Late Transition Metal Oxo Complexes. The Use of Polyoxometalate as a Stabilizing Ligand

Cao, Rui (2008)
Dissertation (290 pages)
Committee Chair / Thesis Adviser: Hill, Craig L
Committee Members: Heaven, Michael ; Edmondson, Dale E
Research Fields: Chemistry, Inorganic
Keywords: X-ray Absorption Spectroscopy; Neutron Diffraction; X-ray Diffraction; Polytungstate; Polyoxometalate; Late Transition Metal Oxo
Program: Laney Graduate School, Chemistry
Permanent url: http://pid.emory.edu/ark:/25593/13x01

Abstract

The use of polyoxometalate (POM) as stabilizing ligands for the synthesis and isolation of late-transition metal oxo (LTMO) complexes is addressed. Metal-oxo species, in particular terminal metal-oxo (O2-) complexes of the late-transition-metal elements have long been thought to exist as transient intermediates in systems ranging from Cu oxidase enzymes to the surfaces of noble metal oxidation catalysts. However, despite decades of speculation and attempted synthesis, no terminal metal-oxo complexes of any element to the right of Ru in the periodic table had been reported prior to 2004 with the exception of the d4 (mesityl)3IrV-oxo complex of G. Wilkinson and co-workers. A current bonding paradigm argues that terminal metal oxo groups are stabilized at metal centers with no more than four d electrons.

This dissertation reports several isolated and fully characterized molecular terminal oxo complexes of the group 10 and 11 elements with the use of polytungstate ligand environments. Polytungstates, which share many structural and reactivity features in common with the metal oxides of broad importance in catalytic technologies (TiO2, CeO2, others), are both good -donating and -accepting ligands that may facilitate stabilization and isolation of terminal late transition metal-oxo units. A total of four structural types of LTMO complexes are presented in this study: (1) M(O)(OH2){A-PW9}2 with one bridging octahedral metal unit between two [A-alfa-PW9O34]9- ligands (M = Pt and Au); (2) M(O)(OH)W(O)(OH2){A-PW9}2 with two linkages between {A-PW9} units, a metal and a tungsten atom (M = Pd); (3) M(O)(OH2)(O=WOH2)2{A-PW9}2 with the terminal M = O group incorporated in a clam shell-like monovacant polytungstate ligand formed by the fusion of two {A-PW9} units by two tungsten atoms (M = Pd and iv Au); and (4) (O=MOH2)2W(O)(OH2){A-PW9}2 (M = Pd), which represents the unique example having two terminal M=O groups coordinated in one molecule. All these molecular LTMO complexes have been carefully studied by geometric and electronic structure methods, including single crystal X-ray diffraction, neutron diffraction, extended X-ray absorption fine structure methods, 31P and 17O NMR spectroscopy, and other chemical and physicochemical methods.

Importantly, the counterion effect in the controlled speciation of the late transition metal substituted polytungstates is discussed. By strongly interacting with the polyanion unit, Cs+ countercations can prevent the hydrolytic decomposition of the tri-metal sandwich structure in solution. As a result, the formation of conventional d8 Pd(II)-substituted polytungstates and unprecedented terminal Pd=O complexes can be controlled.

Reactivity studies are conducted on M(O)(OH2)(O=WOH2)2{A-PW9}2 (M = Pd and Au), a structure that is quite stable in both aqueous and organic solution. The stoichiometric oxo transfer from terminal M=O to other substrates and subsequent reoxidization of the deoxygenated form by air are confirmed by spectroscopy methods. Furthermore, the deoxygenated product, [PdII(O=WOH2)2(A-alfa-PW9O34)2]8-, can be isolated as crystalline plates, and X-ray diffraction confirms the existence of a four-coordinate square-planar Pd(II) center. These results and subsequent catalytic oxidation studies strongly suggest the involvement of terminal M=O species in noble metal-based homogenous and heterogenous catalysts in O2-based green organic oxidations.

Table of Contents

ABSTRACT
ACKNOWLEDGMENT

CHAPTER 1 INTRODUCTION
1.1 TERMINAL TRANSITION METAL OXO UNITS
1.2 SUPPORTED NOBLE METAL CATALYSIS
1.3 BRIDGING NOBLE METAL-OXO SPECIES
1.4 POLYOXOMETALATES - METAL-OXIDE CLUSTERS
1.5 USE OF POLYOXOMETALATES AS STABILIZING LIGAND FOR M=O
REFERENCES

CHAPTER 2 [MO(OH2)(PW9O34)2] (M = PT OR AU)
2.1 ABSTRACT
2.2 INTRODUCTION
2.3 EXPERIMENTAL SECTION
General Methods and Materials
Synthesis of K7Na9[PtO(OH2)(PW9O34)2]21.5H2O (1)
Synthesis of K15H2[AuO(OH2)(PW9O34)2]25H2O (2)
X-ray Crystallographic Studies
Neutron Diffraction Studies
Electrochemistry
X-ray Absorption Spectroscopy Studies
17O NMR Studies
Computational Methods
2.4 RESULTS AND DISCUSSION
Synthesis of Terminal Pt-oxo (1) and Au-oxo (2) complexes
Purity of Complexes, 1 and 2
X-ray crystal structures of 1 and 2
Neutron Diffraction Studies
Optical Spectroscopy
Electrochemistry
X-ray Absorption Spectroscopy Studies
17O NMR studies
Computational Studies
REFERENCES

CHAPTER 3 [MO(OH)WO(OH2)(PW9O34)2] (M = PD)
3.1 ABSTRACT
3.2 INTRODUCTION
3.3 EXPERIMENTAL SECTION
General Methods and Materials
Synthesis of H5.6K7.6Na1.4[Pd0.3{WO(OH2)}0.7(PW9O34)2]
Synthesis of K10Na3[PdO(OH)WO(OH2)(PW9O34)2]16H2O (3)
Crystallographic Studies
Titration Studies
Electronic Absorption Studies
17O NMR Studies
X-ray Absorption Studies
Electrochemistry
Computational Procedures
3.4 RESULTS AND DISCUSSION
Synthesis of K10Na3[PdO(OH)WO(OH2)(PW9O34)2]16H2O (3)
Crystallographic Studies
X-ray Absorption Studies
Titration Studies
17O NMR Studies
Electronic Absorption Studies
Electrochemistry
Pd(IV) oxidation state assignment
Computational Results
REFERENCES

CHAPTER 4 [MO(OH2)(WO(OH2))2(PW9O34)2] (M = PD OR AU)
4.1 ABSTRACT
4.2 INTRODUCTION
4.3 EXPERIMENTAL SECTION
General Methods and Materials
Synthesis of K8[PdIVO(OH2)P2W20O70(OH2)2]22H2O (4)
Synthesis of K7H2[AuIIIO(OH2)P2W20O70(OH2)2]27H2O (5)
X-ray Crystallographic Studies
X-ray Absorption Spectroscopy Studies
Ultra-Low-Temperature Electronic Absorption Spectroscopy
pH Dependent UV-vis and 31P NMR Titrations
Electrochemistry
Chemical Titrations
17O NMR Studies
4.4 RESULTS AND DISCUSSION
Synthesis
Magnetism of Au LTMO complexes, 2 and 5
X-ray Crystallographic Studies
X-ray Absorption Spectroscopy Studies
Ultra-Low-Temperature Electronic Absorption Spectroscopy
pH Dependent UV-vis and 31P NMR Titrations
Electrochemistry
Chemical Titrations
17O NMR Studies
Atomic Occupancy at the Au Position
Au Oxidation State
4.5 CONCLUSIONS
REFERENCES

CHAPTER 5 [(MO(OH2))2WO(OH2)(PW9O34)2] (M = PD)
5.1 ABSTRACT
5.2 INTRODUCTION
5.3 EXPERIMENTAL SECTION
General Methods and Materials
Synthesis of Cs3.5K3Na3.5[(O=PdIV(OH2))2P2W19O68(OH)2]20H2O (6)
Crystallography Studies
X-ray Absorption Spectroscopy
Reduction of 6 with Na2SO3 and (CH3)2S
Computational Studies
5.4 RESULTS AND DISCUSSION
Synthesis
Thermogravimetric Analysis (TGA)
Structural Studies
Pd(IV) Oxidation State Assignment
Reduction of 6
Computational Studies
REFERENCES

CHAPTER 6 COUNTERCATION EFFECT
6.1 ABSTRACT
6.2 INTRODUCTION
6.3 EXPERIMENTAL SECTION
General Methods and Materials
Synthesis of K12[PdII 3(PW9O34)2]18H2O (K9)
Synthesis of Cs8Na4[PdII 3(PW9O34)2]18H2O (CsNa9)
Synthesis of Cs9Na5[PdII 3(SiW9O34)2]16H2O (10)
Synthesis of Cs5K3Na4[PdII 2WO(OH2)(SiW9O34)2]16H2O (11)
Crystallographic Studies
6.4 RESULTS AND DISCUSSION
Synthesis of K9 and CsNa9
Effect of Cs Countercation in the Synthesis, Controlled Speciation of 10 and 11
Crystallographic Studies of K9, CsNa9, 10 and 11
6.5 CONCLUSION
REFERENCES

CHAPTER 7 REACTIVITY STUDIES
7.1 ABSTRACT
7.2 INTRODUCTION
7.3 EXPERIMENTAL SECTION
General Methods and Materials
Stoichiometric Oxo Transfer of Au-oxo Complex 5
Stoichiometric Oxo Transfer of Pd-oxo Complex 4
Crystallography Studies of the Deoxygenated Product 12
Reoxygenation of Pd(II) Complex 12
Catalytic Properties of Pd-oxo Complex 4
Comparative Reaction: [PdIIBr4]2- + Ph3P
7.4 RESULTS AND DISCUSSION43
Stoichiometric Oxo Transfer of Au-oxo Complex 5
Stoichiometric Oxo Transfer of Pd-oxo Complex 4
Crystallography Studies of 12
Reoxygenation of Pd(II) Complex 12
Catalytic Oxidation by Pd-oxo Complex 4
Comparative Reactions
REFERENCES

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